The glycine transport inhibitor sarcosine is an NMDA receptor co-agonist that differs from glycine

Sarcosine is an amino acid involved in one-carbon metabolism and a promising therapy for schizophrenia because it enhances NMDA receptor (NMDAR) function by inhibiting glycine uptake. The structural similarity between sarcosine and glycine led us to hypothesize that sarcosine is also an agonist like glycine. We examined this possibility using whole-cell recordings from cultured embryonic mouse hippocampal neurons. We found that sarcosine is an NMDAR co-agonist at the glycine binding site. However, sarcosine differed from glycine because less NMDAR desensitization occurred with sarcosine than with glycine as the co-agonist. This finding led us to examine whether the physiological effects of NMDAR activation with these two co-agonists are the same. The difference in desensitization probably accounts for rises in intracellular Ca²⁺, as assessed by the fluorescent indicator fura-FF, being larger when NMDAR activation occurred with sarcosine than with glycine. In addition, Ca²⁺-activated K⁺ currents following NMDAR activation were larger with sarcosine than with glycine. Compared to glycine, NMDAR-mediated autaptic currents decayed faster with sarcosine suggesting that NMDAR deactivation also differs with these two co-agonists. Despite these differences, NMDAR-dependent neuronal death as assessed by propidium iodide was similar with both co-agonists. The same was true for neuronal bursting. Thus, sarcosine may enhance NMDAR function by more than one mechanism and may have different effects from other NMDAR co-agonists.     

Tonic NMDA receptor-mediated current in prefrontal cortical pyramidal cells and fast-spiking interneurons

Tonically activated neuronal currents mediated by N-methyl-d-aspartate receptors (NMDARs) have been hypothesized to contribute to normal neuronal function as well as to neuronal pathology resulting from excessive activation of glutamate receptors (e.g., excitotoxicity). Whereas cortical excitatory cells are very vulnerable to excitotoxic insult, the data regarding resistance of inhibitory cells (or interneurons) are inconsistent. Types of neurons with more pronounced tonic NMDAR current potentially associated with the activation of extrasynaptic NMDARs could be expected to be more vulnerable to excessive activation by glutamate. In this study, we compared tonic activation of NMDARs in excitatory pyramidal cells and inhibitory fast-spiking interneurons in prefrontal cortical slices. We assessed tonic NMDAR current by measuring holding current shift as well as noise reduction following NMDAR blockade after removal of spontaneous glutamate release. In addition, we compared NMDAR miniature excitatory postsynaptic currents (EPSCs) in both cell types. We have demonstrated for the first time that tonic NMDAR currents are present in inhibitory fast-spiking interneurons. We found that the magnitude of tonic NMDAR current is similar in pyramidal cells and fast-spiking interneurons, and that quantal release of glutamate does not significantly impact tonic NMDAR current.     

Sex-dependent differences in the activity and modulation of N-methyl-d-aspartic acid receptors in rat dorsal root ganglia neurons

Women have greater temporal summation of experimental pain stimuli and also have a higher propensity for developing chronic visceral pain conditions. Sex hormone-mediated regulation of N-methyl-d-aspartic acid receptors (NMDARs) in nociceptive pathways is a plausible mechanism that may underlie these phenomena. The aim of this study was to compare the effect of 17-beta-estradiol (E2) in modulation of NMDAR activity in adult male and female rat dorsal root ganglia (DRG) neurons. DRG neurons were collected from adult male or female rats and grown in short-term culture in steroid-free media. NMDAR currents were recorded on small to medium size neurons by whole cell patch clamp using rapid perfusion with saturating concentrations of N-methyl-d-aspartic acid and glycine in the absence of extracellular Mg(2+). We found that the average density of NMDAR currents was 2.8-fold larger in DRG neurons from female rats compared with male rats (P<0.0001). Addition of 100 nM E2 increased NMDAR currents 55+/-15% in female neurons, but only 19+/-7% in male neurons. Potentiation was maximal after 20-40 min and dose dependent with an apparent 50% excitatory concentration of 17-23 nM. This effect was mimicked by E2 conjugated to BSA and attenuated by pretreatment with the protein tyrosine kinase inhibitor lavendustin A (1 microM) or the estrogen receptor (ER) antagonist, ICI 182,780 (1 microM), strongly suggesting activation of a cell surface ER acting through a non-genomic mechanism involving protein tyrosine kinases to increase NMDAR currents. These results identify sex-based differences in both the basal expression and the regulation of the NMDARs in DRG neurons.     

Glycine transporter type 1 blockade changes NMDA receptor-mediated responses and LTP in hippocampal CA1 pyramidal cells by altering extracellular glycine levels

Long-term potentiation (LTP) in the hippocampal CA1 region requires the activation of NMDA receptors (NMDARs). NMDAR activation in turn requires membrane depolarization as well as the binding of glutamate and its coagonist glycine. Previous pharmacological studies suggest that the glycine transporter type 1 (GlyT1) maintains subsaturating concentrations of glycine at synaptic NMDARs. Antagonists of GlyT1 increase levels of glycine in the synaptic cleft and, like direct glycine site agonists, can augment NMDAR currents and NMDAR-mediated functions such as LTP. In addition, stimulation of the glycine site initiates signalling through the NMDAR complex, priming the receptors for clathrin-dependent endocytosis. We have used a new potent GlyT1 antagonist, CP-802,079, with whole-cell patch-clamp recordings in acute rat hippocampal slices to determine the effect of GlyT1 blockade on LTP. Reverse microdialysis experiments in the hippocampus of awake, freely moving rats, showed that this drug elevated only the extracellular concentration of glycine. We found that CP-802,079, sarcosine and glycine significantly increased the amplitude of the NMDAR currents and LTP. In contrast, application of higher concentrations of CP-802,079 and glycine slightly reduced NMDAR currents and did not increase LTP. Overall, these data suggest that the level of glycine present in the synaptic cleft tightly regulates the NMDAR activity. This level is kept below the 'set point' of the NMDAR internalization priming mechanism by the presence of GlyT1-dependent uptake.     

Effect of d-serine on the serotonin receptors of human platelets

Recent literature and our previous observations indicated the presence of both NMDA and serotonin type 3 receptors in human platelets with very similar ionic currents to that of cultured mammalian neuronal receptors. Baseline electrophysiological data shows similar profile for platelets from both normal and schizophrenic subjects, whereas serotonin receptor studies exhibited the presence of 5-hydroxytryptamine type-3 (5-HT3) currents in both normal and schizophrenic platelets significantly different from each other. The two major differences observed were: first, 5-HT3 receptors present in the platelets of schizophrenic patients were four times more sensitive to serotonin than those present in the platelets of normal subjects and, second, that d-serine in micro molar concentrations dampens this effect in platelets from schizophrenic patients but increases the sensitivity of serotonin for platelet 5-HT3 receptors of normal subjects. Patch clamp technique was used to measure the whole cell currents passing through serotonin receptors in these two types of human platelets. The currents were found to be 5-HT3 receptor currents as they were abolished by 10 μM d-tubocurarine. Similarly, micromolar concentrations of d-serine increased the sensitivity of 5HT3 receptor currents in the normal human platelets but decreased it in the platelets of the schizophrenic patients. This effect was reversed when d-amino acid oxidase (0.3 μM) was co applied with 100 μM of d-serine, raising the possibility that d-serine by itself may act as a modulator for platelet 5-HT3 receptor channel currents. These observations raised exciting new questions about the role of platelet serotonin receptors and their regulation by d-serine.     

Cross talk between synaptic receptors mediates NMDA-induced suppression of inhibition

Past research has shown that calcium influx through NMDA receptors (NMDARs) depresses GABA(A) currents. We examined upstream triggers of this suppression, including involvement of target synaptic GABA(A) receptors and the NMDARs triggering suppression. In hippocampal neurons, conditioning with 20 μM NMDA for 20 s caused 50% suppression of GABA responses. The suppression was delayed by ≈ 60 s following NMDA application and persisted for at least 5 min following conditioning. Pharmacology experiments suggested a shift in both the sensitivity to GABA and a loss of functional receptors. NMDA conditioning strongly suppressed inhibitory postsynaptic currents and speeded decay kinetics. Synaptic NMDAR conditioning was necessary to suppress GABA current in pyramidal neurons; extrasynaptic NMDAR activation did not suppress, even when matched to synaptic activation. We found no evidence that specific synaptic NMDAR subunits mediate depression of GABA responses. Although physical colocalization of glutamate and GABA(A) receptors is mostly likely in extrasynaptic regions, our evidence suggests that NMDAR-induced suppression of GABA responsiveness prominently affects precise, moment-to-moment signaling from synaptic receptors to synaptic receptors.     

NMDA receptor desensitization regulated by direct binding to PDZ1-2 domains of PSD-95

Regulation of N-methyl-d-aspartate receptor (NMDAR) activity by desensitization is important in physiological and pathological states; NMDAR desensitization contributes in shaping synaptic responses and may be protective by limiting calcium influx during sustained glutamate insults. We previously reported that glycine-independent desensitization decreases during hippocampal neuronal development, correlating with NMDAR synaptic localization and association with postsynaptic density 95 (PSD-95). PSD-95/Discs large/zona occludens (PDZ)-1,2 domains of PSD-95 bind to the C-terminus of NMDAR NR2 subunits. The role of PSD-95 in anchoring signaling proteins near NMDARs is well documented. To determine if PSD-95-induced changes in NMDAR desensitization occur because of direct binding to NR2 or due to recruitment of regulatory proteins, we tested the effects of various PSD-95 constructs on NMDAR currents in human embryonic kidney 293 (HEK293) cells and neurons. In HEK cells, wild-type PSD-95 significantly reduced wild-type NMDAR desensitization without altering currents of NMDARs containing NR2A-S1462A, a mutation that abolishes PSD-95 binding. The PSD-95 N-terminus truncated after the PDZ1-2 domains was sufficient for this effect in neurons with low endogenous PSD-95 levels; in NMDAR-expressing HEK cells, the effect persisted when PSD-95 multimerization was eliminated. Moreover other PSD-95 family members with highly homologous PDZ1-2 domains significantly reduced NMDAR desensitization. In mature neurons, disruption of PSD-95/NMDAR interaction through protein kinase C (PKC) activation increased desensitization to levels found in immature neurons, and this effect was not due to PKC direct regulation of NMDAR activity. We conclude that direct binding of PSD-95 increases stability of NMDAR responses to agonist exposure in neuronal and nonneuronal cells.     

Regional variations in the glial influence on synapse development in the mouse CNS

There is increasing evidence that synapse function depends on interactions with glial cells, namely astrocytes. Studies on specific neurons of the central nervous system (CNS) indicated that glial signals also control synapse development, but it remained unclear whether this is a general principle that applies to other neuronal cell types. To address this question, we developed new methods to immunoisolate neurons from different brain regions of postnatal mice and to culture them in a chemically defined medium. Electrophysiological recordings and immunocytochemical staining revealed vigorous synaptogenesis in hippocampal and cerebellar neurons, but not in retinal ganglion cells (RGCs) in the absence of glial cells. Co-culture with glia promoted synapse formation in RGCs as indicated by a strong increase in the incidence and frequency of action potential-independent miniature synaptic currents, but showed no such effects in hippocampal or cerebellar neurons. On the other hand, glial signals promoted the efficacy of excitatory synapses in all regions as indicated by an increase in the size of spontaneous synaptic events in cerebellar cultures and of miniature synaptic currents in hippocampal neurons and RGCs. Inhibitory synaptic currents remained largely unaffected by glia. Our results indicate that in the mammalian CNS, the way that glial signals promote the development of excitatory synapses depends on the type of neuron.     

NMDA receptor activation enhances inhibitory GABAergic transmission onto hippocampal pyramidal neurons via presynaptic and postsynaptic mechanisms

N-methyl-D-aspartate (NMDA) receptors (NMDARs) are implicated in synaptic plasticity and modulation of glutamatergic excitatory transmission. Effect of NMDAR activation on inhibitory GABAergic transmission remains largely unknown. Here, we report that a brief application of NMDA could induce two distinct actions in CA1 pyramidal neurons in mouse hippocampal slices: 1) an inward current attributed to activation of postsynaptic NMDARs; and 2) fast phasic synaptic currents, namely spontaneous inhibitory postsynaptic currents (sIPSCs), mediated by GABA(A) receptors in pyramidal neurons. The mean amplitude of sIPSCs was also increased by NMDA. This profound increase in the sIPSC frequency and amplitude was markedly suppressed by the sodium channel blocker TTX, whereas the frequency and mean amplitude of miniature IPSCs were not significantly affected by NMDA, suggesting that NMDA elicits repetitive firing in GABAergic interneurons, thereby leading to GABA release from multiple synaptic sites of single GABAergic axons. We found that the NMDAR open-channel blocker MK-801 injected into recorded pyramidal neurons suppressed the NMDA-induced increase of sIPSCs, which raises the possibility that the firing of interneurons may not be the sole factor and certain retrograde messengers may also be involved in the NMDA-mediated enhancement of GABAergic transmission. Our results from pharmacological tests suggest that the nitric oxide signaling pathway is mobilized by NMDAR activation in CA1 pyramidal neurons, which in turn retrogradely facilitates GABA release from the presynaptic terminals. Thus NMDARs at glutamatergic synapses on both CA1 pyramidal neurons and interneurons appear to exert feedback and feedforward inhibition for determining the spike timing of the hippocampal microcircuit.     

Prolonged NMDA-mediated responses, altered ifenprodil sensitivity, and epileptiform-like events in the malformed hippocampus of methylazoxymethanol exposed rats

Cortical malformations are often associated with refractory epilepsy and cognitive deficit. Clinical and experimental studies have demonstrated an important role for glutamate-mediated synaptic transmission in these conditions. Using whole cell voltage-clamp techniques, we examined evoked glutamate-mediated excitatory postsynaptic currents (eEPSCs) and responses to exogenously applied glutamate on hippocampal heterotopic cells in an animal model of malformation i.e., rats exposed to methylazoxymethanol (MAM) in utero. Analysis revealed that the late N-methyl-D-aspartate (NMDA) receptor-mediated eEPSC component was significantly increased on heterotopic cells compared with age-matched normotopic pyramidal cells. At a holding potential of +40 mV, heterotopic cells also exhibited eEPSCs with a slower decay-time constant. No differences in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component of eEPSCs were detected. In 23% of heterotopic pyramidal cells, electrical stimulation evoked prolonged burst-like responses. Focal application of glutamate (10 mM) targeted to different sites near the heterotopia also evoked epileptiform-like bursts on heterotopic cells. Ifenprodil (10 microM), an NR2B subunit antagonist, only slightly reduced the NMDA receptor (NMDAR)-mediated component and amplitude of eEPSCs on heterotopic cells (MAM) but significantly decreased the late component and peak amplitude of eEPSCs in normotopic cells (control). Our data demonstrate a functional alteration in the NMDA-mediated component of excitatory synaptic transmission in heterotopic cells and suggest that this alteration may be attributable, at least in part, to changes in composition and function of the NMDAR subunit. Changes in NMDAR function may directly contribute to the hyperexcitability and cognitive deficits reported in animal models and patients with brain malformations.     

AMPA receptor subunit GluR1 (GluA1) serine-845 site is involved in synaptic depression but not in spine shrinkage associated with chemical long-term depression

The structure of dendritic spines is highly plastic and can be modified by neuronal activity. In addition, there is evidence that spine head size correlates with the synaptic α-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA) receptor (AMPAR) content, which suggests that they may be coregulated. Although there is evidence that there are overlapping mechanisms for structural and functional plasticity, the extent of the overlap needs further investigation. Specifically, it is unknown whether AMPAR levels determine spine size or whether both are regulated via parallel pathways. We studied the correlation between spine structural plasticity and long-term synaptic plasticity following chemical-induced long-term depression (chemLTD). In particular, we examined whether the regulation of AMPARs, which is implicated in LTD, is critical for spine morphological plasticity. We used mutant mice specifically lacking the serine-845 site on the type 1 glutamate receptor (GluR1, or GluA1) subunit of AMPARs (mutants). These mice specifically lack N-methyl-D-aspartate (NMDA) receptor (NMDAR)-dependent LTD and NMDAR activation-induced AMPAR endocytosis. We found that chemLTD causes a rapid and persistent shrinkage in spine head volume of hippocampal CA1 pyramidal neurons in wild types similar to that reported in other studies using low-frequency stimulation (LFS)-induced LTD. Surprisingly, we found that although S845A mutant mice display impaired chemLTD, the shrinkage of spine head volume occurred to a similar magnitude to that observed in wild types. Our results suggest that there is dissociation in the molecular mechanisms underlying functional LTD and spine shrinkage and that GluR1-S845 regulation is not necessary for spine morphological plasticity.     

培养海马神经元突触外NMDA受体通道电流在发育中的变化

为观察培养大鼠海马神经元突触外NMDA受体通道电流在发育中的变化,本研究采用膜外面向外模式记录突触外NMDA受体介导的单通道电流。结果显示:培养2周神经元的电流幅度和开放概率比培养1周神经元大,但电导和翻转电位无显著差异。培养2周神经元只出现高电导开放,培养1周神经元同时出现高电导和低电导两种开放形式。NR2B受体亚型的特异性拮抗剂ifenprodil可降低培养1周和2周海马神经元的电流幅度、电导和开放概率,且对培养2周神经元开放概率的抑制作用更明显。以上结果表明,培养海马神经元突触外NMDA受体通道电流有发育变化,且培养1周神经元突触外NMDA受体的NR2亚型可能为NR2B和NR2D;而神经元培养到2同时,突触外主要为NR2B亚型,且数量有所增加。     

Early modifications in N-methyl-d-aspartate receptor subunit mRNA levels in an oxygen and glucose deprivation model using rat hippocampal brain slices

Glutamatergic N-methyl-d-aspartate NMDA receptors (NMDAR) are considered to play a key role in ischemia-induced damage. Long-term (hours) changes in their expression upon ischemia have been shown. Here we report short-term changes in the mRNA levels of the major hippocampal NMDAR subunits (NR1, NR2A and NR2B), as well as c-fos, in an ex vivo ischemia model using hippocampal slices. This effect can be observed also in a calcium free incubation solution. Striking early decreases in the NMDAR subunit mRNA levels were observed after 30 min of oxygen and glucose deprivation (OGD) as well as a partial recovery when the tissues were returned to the balanced salt solution (reperfusion-like period) for 3 h. Since OGD-induced damage has been reported to be a consequence of the increase in OGD-related glutamate release, we also analyzed NMDAR mRNA levels following increased glutamate levels in hippocampal sections in which no significant effects on NMDAR subunit mRNA levels were detected. Furthermore, we describe that the presence of MK-801 (a selective NMDAR antagonist), CNQX (a selective AMPA/kainate receptor antagonist) or their combined action in the incubation solution is able to induce a significant decrease in NMDAR expression but in these conditions the OGD does not induce further decreases in mRNA levels. We suggest that the mechanisms triggered during OGD to downregulate mRNA levels of NMDAR subunits could be the same than those induced by glutamate receptor antagonists.     

Cell type dependence and variability in the short-term plasticity of EPSCs in identified mouse hippocampal interneurones

Synapses exhibit different short-term plasticity patterns and this behaviour influences information processing in neuronal networks. We tested how the short-term plasticity of excitatory postsynaptic currents (EPSCs) depends on the postsynaptic cell type, identified by axonal arborizations and molecular markers in the hippocampal CA1 area. Three distinct types of short-term synaptic behaviour (facilitating, depressing and combined facilitating-depressing) were defined by fitting a dynamic neurotransmission model to the data. Approximately 75 % of the oriens-lacunosum-moleculare (O-LM) interneurones received facilitating EPSCs, but in three of 12 O-LM cells EPSCs also showed significant depression. Over 90 % of the O-LM cells were immunopositive for somatostatin and mGluR1α and all tested cells were decorated by strongly mGluR7a positive axon terminals. Responses in eight of 12 basket cells were described well with a model involving only depression, but the other cells displayed combined facilitating-depressing EPSCs. No apparent difference was found between the plasticity of EPSCs in cholecystokinin- or parvalbumin-containing basket cells. In oriens-bistratified cells (O-Bi), two of nine cells showed facilitating EPSCs, another two depressing, and the remaining five cells combined facilitating-depressing EPSCs. Seven of 10 cells tested for somatostatin were immunopositive, but mGluR1α was detectable only in two of 11 tested cells. Furthermore, most O-Bi cells projected to the CA3 area and the subiculum, as well as outside the hippocampal formation. Postsynaptic responses to action potentials recorded in vivo from a CA1 place cell were modelled, and revealed great differences between and within cell types. Our results demonstrate that the short-term plasticity of EPSCs is cell type dependent, but with significant heterogeneity within all three interneurone populations.     

Glycine and glycine receptor signaling in hippocampal neurons: Diversity,function and regulation

Glycine is a primary inhibitory neurotransmitter in the spinal cord and brainstem. It acts at glycine receptor (GlyR)-chloride channels, as well as a co-agonist of NMDA receptors (NMDARs). In the hippocampus, the study of GlyRs has largely been under-appreciated due to the apparent absence of glycinergic synaptic transmission. Emerging evidence has shown the presence of extrasynaptic GlyRs in the hippocampus, which exert a tonic inhibitory role, and can be highly regulated under many pathophysiological conditions. On the other hand, besides d-serine, glycine has also been shown to modulate NMDAR function in the hippocampus. The simultaneous activation of excitatory NMDARs and inhibitory GlyRs may provide a homeostatic regulation of hippocampal network function. Furthermore, glycine can regulate hippocampal neuronal activity through GlyR-mediated cross-inhibition of GABAergic inhibition, or through the glycine binding site-dependent internalization of NMDARs. Therefore, hippocampal glycine and its receptors may operate in concert to finely regulate hippocampus-dependent high brain function such as learning and memory. Finally, dysfunction of hippocampal glycine signaling is associated with neuropsychiatric disorders. We speculate that further studies of hippocampal glycine-mediated regulation may help develop novel glycine-based approaches for therapeutic developments.     

NMDA receptor-dependent metaplasticity at hippocampal mossy fiber synapses

Hippocampal mossy fiber synapses have been reported to lack NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of AMPA excitatory postsynaptic currents (EPSCs), unlike conventional glutamatergic synapses. An explanation for this difference may reside in the relatively low number of NMDARs at these synapses. Because mossy fiber synapses display LTP selective for NMDARs, we examined whether this would affect the plasticity rules at mossy fiber-CA3 synapses in mouse hippocampal slices. We found that LTP of NMDARs serves as a metaplastic switch making mossy fiber synapses competent for generating NMDAR-dependent LTP of AMPA EPSCs.     

Distribution of NMDA receptor subunit NR1 in Arctic ground squirrel central nervous system

Hibernation is a natural model of neuroprotection and adult synaptic plasticity. NMDA receptors (NMDAR), which play key roles in excitotoxicity and synaptic plasticity, have not been characterized in a hibernating species. Tolerance to excitotoxicity and cognitive enhancement in Arctic ground squirrels (AGS, Spermophilus parryii) suggests that NMDAR expression may decrease in hibernation and increase upon arousal. NMDAR consist of at least one NMDAR1 (NR1) subunit, which is required for receptor function. Localization of NR1 reflects localization of the majority, if not all, NMDAR complexes. The purpose of this study, therefore, was to characterize the distribution of NR1 subunits in AGS central nervous system using immunohistochemistry. In addition, we compare NR1 expression in hippocampus of hibernating AGS (hAGS) and inter-bout euthermic AGS (ibeAGS) and assess changes in cell somata size using NR1 stained sections in three hippocampal sub-regions (CA1, CA3, and dentate gyrus). For the first time, we report that immunoreactivity of anti-NR1 is widely distributed throughout the central nervous system in AGS and is similar to other species. No differences exist in the expression and distribution of NR1 in hAGS and ibeAGS. However, we report a significant decrease in size of hippocampal CA1 and dentate gyrus NR1-expressing neuronal somata during hibernation torpor.     

The NMDA-to-AMPA ratio at synapses onto layer 2/3 pyramidal neurons is conserved across prefrontal and visual cortices

To better understand regulation of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor complements across the cortex, and to investigate NMDA receptor (NMDAR)-based models of persistent activity, we compared NMDA/AMPA ratios in prefrontal (PFC) and visual cortex (VC) in rat. Whole cell voltage-clamp responses were recorded in brain slices from layer 2/3 pyramidal cells of the medial PFC and VC of rats aged p16-p21. Mixed miniature excitatory postsynaptic currents (mEPSCs) having AMPA receptor (AMPAR)- and NMDAR-mediated components were isolated in nominally 0 Mg2+ ACSF. Averaged mEPSCs were well-fit by double exponentials. No significant differences in the NMDA/AMPA ratio (PFC: 27 +/- 1%; VC: 28 +/- 3%), peak mEPSC amplitude (PFC: 19.1 +/- 1 pA; VC: 17.5 +/- 0.7 pA), NMDAR decay kinetics (PFC: 69 +/- 8 ms; VC: 67 +/- 6 ms), or degree of correlation between NMDAR- and AMPAR-mediated mEPSC components were found between the areas (PFC: n = 27; VC: n = 28). Recordings from older rats (p26-29) also showed no differences. EPSCs were evoked extracellularly in 2 mM Mg2+ at depolarized potentials; although the average NMDA/AMPA ratio was larger than that observed for mEPSCs, the ratio was similar in the two regions. In nominally 0 Mg2+ and in the presence of CNQX, spontaneous activation of NMDAR increased recording noise and produced a small tonic depolarization which was similar in both areas. We conclude that this basic property of excitatory transmission is conserved across PFC and VC synapses and is therefore unlikely to contribute to differences in firing patterns observed in vivo in the two regions.